a) Field of the Invention
The invention is directed to an arrangement for providing a reproducible target flow for the energy beam-induced generation of a plasma that emits a short-wavelength radiation, in particular for the generation of EUV radiation. It is applied particularly in projection lithography for semiconductor chip fabrication.
b) Description of the Related Art
A radiation source based on energy beam-induced excitation of plasma that is used for applications which are stable over long periods of time, e.g., in semiconductor fabrication for EUV lithography, must have a very durable injection system for providing targets so that the required high directional stability is maintained over a very large number of individual plasma generation processes.
Systematic studies conducted by XTREME technologies GmbH on the operating life of target nozzles have shown that erosion at a nozzle after approximately one million plasma generation processes leads to an unstable target flow. Sputter particles (ions or atoms) that are unavoidably emitted by the plasma along with the desired radiation have been determined as the cause of erosion at the nozzle opening.
In energy beam-induced XUV plasmas (in particular laser-excited EUV plasmas), mass-limited targets, i.e., targets that provide the approximate quantity of atoms that can be excited to radiation in the region of interaction with the energy beam, are used according to the prior art. Mass-limited targets of this kind, which are preferably droplet target flows or jet target flows with a diameter appreciably less than one millimeter (at least in one dimension), have the purpose of minimizing vaporization processes of target material that is not sufficiently excited and minimizing the generation of sputter particles (usually called debris) in the interaction chamber. However, the generation of debris from the plasma cannot be totally suppressed.
In prior art EUV radiation sources, these kinds of sputter effects from the plasma have obviously not yet been investigated in relation to the target nozzle in view of the fact that publications concerning debris reduction are geared exclusively to the operating life of the optics that are employed. For example, WO 99/42904 discloses a filter for protecting collector optics which is positioned between the source and the optics as a honeycomb structure. The interaction of the particles with a background gas results in a retardation of the particles and subsequent condensation at the filter walls. However, since the filter is arranged in the optical light path of the emitted radiation, a sufficiently high degree of transparency must be ensured for the emitted EUV radiation in the interaction chamber over a large solid angle by a sufficiently low (vacuum) pressure on the one hand, so that the gas atmosphere in the interaction chamber absorbs as little emitted radiation as possible, and, on the other hand, by minimizing shadows cast by the honeycomb structure of the filter.
As will be shown, solutions are also known from the prior art which have similarities to the present invention, although the teaching according to the invention is not rendered obvious thereby.
US 2003/0223546, for example, describes a pressure reservoir directly around the target nozzle with a buffer gas which serves to generate droplet targets and which especially reinforces the target shaping of xenon droplets. Another surrounding chamber in which the buffer gas can then be sucked out again leads to an acceleration of the droplet targets and regulation of the intervals between the droplet targets. There is no mention of a reaction on the target nozzle.
Further, it has been shown for a radiation source with high average output such as is required in semiconductor chip fabrication that degradation processes likewise occur at the nozzle due to the radiation from the plasma that is absorbed by the nozzle. By degradation is meant both irreversible and reversible thermal changes due to radiation absorption at the nozzle opening which lead—at least temporarily—to an appreciable deterioration in the directional stability of the target jet.
With regard to the set of problems in stabilizing a reproducibly provided, continuous target jet over time, WO 97/40650 discloses a step in which a continuous target jet is provided as a stable target flow for short-wavelength radiation sources. However, the problem of decreasing jet stability due to nozzle erosion over longer operating periods is not examined. Therefore, there is also no indication of suitable countermeasures.
It is the object of the invention to find a novel possibility for providing a reproducibly supplied target flow for the generation of a plasma that emits short-wavelength radiation which ensures a high directional stability of the target flow over a large number of individual plasma generation process for target materials with any vapor pressure under given process conditions.
In an arrangement for providing a reproducible target flow for the energy beam-induced generation of a plasma emitting short-wavelength radiation, particularly for the generation of EUV radiation in which a target nozzle is provided for introducing target material under pressure into an interaction chamber and in which an energy beam is directed to the target flow at an interaction point in the interaction chamber, the above-stated object is met, according to the invention, in that a nozzle protection device is provided in the interaction chamber between the target nozzle and the interaction point for the generation of the plasma, and in that the nozzle protection device contains a gas pressure chamber which has an aperture along the target path for unobstructed passage of the target flow and which is filled with a buffer gas that is maintained under a pressure at which a sputter particle from the plasma is subjected to at least one thousand collisions with particles of the buffer gas when traversing the gas pressure chamber.
In a pressure range of some 10 mbar, an energy sputter particle already collides with particles of the buffer gas several thousand times over a distance of a few millimeters through the gas pressure chamber and loses several orders of magnitude of kinetic energy. The person skilled in the art will immediately be led to variations of longer gas pressure chambers with lower buffer gas pressures.
The nozzle protection device is advantageously constructed as a sputter protection plate in which the gas pressure chamber is incorporated. The gas pressure chamber has a cylindrical aperture and, radially, at least one channel for supplying the buffer gas.
In a first variant, the sputter protection plate advisably has a plurality of uniformly distributed radial channels as gas feeds for the buffer gas and an annular distribution channel arranged concentrically around the gas pressure chamber. The annular distribution channel connects the radial channels and has at least one gas inlet opening that does not meet one of the radial channels.
In a second variant, the sputter protection plate advantageously has an upper terminating plate and a lower terminating plate, each with an aperture for the passage of the target flow. The terminating plates are connected parallel to one another by an annular distribution channel which has at least one inlet opening for gas supply. The apertures of the gas pressure chamber are advisably arranged in the preferably circular terminating plates as coaxial bore holes.
It has proven advantageous when the nozzle protection device additionally has a heat protection plate with coolant channels or the coolant channels are integrated in the material of the gas pressure chamber as heat protection.
The nozzle protection device with the gas pressure chamber is advantageously arranged in the interaction chamber at a defined distance from the target nozzle.
In another advisable construction, the gas pressure chamber is arranged in the interaction chamber directly around the target nozzle. It is advantageous when the gas pressure chamber is arranged around the opening of the target nozzle by means of an antechamber housing that surrounds the target nozzle in a gas-tight manner. The antechamber housing has an aperture that is centered with respect to the axis of the target flow and has at least one gas feed for providing the buffer gas.
With respect to the injected target flow, a tin is advantageously used as the main target material and can liquefy under necessary defined process conditions. Tin chlorides, preferably tin(IV) chloride or tin(II) chloride in alcoholic or aqueous solution, are particularly suitable for this purpose.
An inert gas is advisably used as buffer gas for generating a partial pressure in the gas pressure chamber. This inert gas can be nitrogen or any noble gas, preferably argon. On the other hand, mixtures of inert gases can also be used, particularly a mixture of noble gases such as helium and neon.
In a particular construction of the nozzle protection device for which target materials with a vapor pressure of >50 mbar are used, the buffer gas in the gas pressure chamber is formed by gaseous target material due to the vaporization of the target flow in the interaction chamber and a partial pressure of some 10 mbar is adjusted due to the flow of vaporizing target material through the gas pressure chamber. This obviates a separate supply of buffer gas.
For this embodiment form of the invention, preferably liquid xenon is injected through the target nozzle as target material.
To support the buildup of pressure through vaporizing target material, the gas pressure chamber advantageously has at least one narrowed aperture for generating a dynamic pressure. The gas pressure chamber is preferably barrel-shaped.
The underlying idea of the invention is based on the understanding that the target nozzle (as well as the collector optics) of a plasma-based radiation source is damaged by debris emission and radiation from the plasma. However, for nozzle protection, in contrast to optics, a high optical transparency is not required. Rather, other parameters apply for optimal protection of the nozzle which merely do not impede or interfere with the liquid target flow. Therefore, the invention does not use a filter, but rather employs a gas pressure chamber which is arranged between the target nozzle and plasma along the target path with an individual aperture and in which the target nozzle is shielded from fast debris particles and from radiation emitted by the plasma by a quasi-statically adjusted, relatively high buffer gas pressure (some 10 mbar compared to the vacuum of less than 1 mbar in the interaction chamber).
The solution according to the invention makes it possible to provide a reproducibly supplied target flow for the generation of a plasma emitting short-wavelength radiation which ensures a high directional stability of the target flow for target materials with any vapor pressure under the respective process conditions over a large number of plasma generation processes and which therefore makes it possible to produce radiation sources with a long operating life.
The invention will be described more fully in the following with reference to embodiment examples.